Integrated visible photonics has become a vastly growing area in optics [1]} due to emerging applications such as atomic clocks [2]}, quantum information processing with quantum dots [3]}, trapped ions and atom-like color centers [4]}, [5]}, [6]}, bio-sensing [7]} and so on. Among many integrated photonic material platforms [8]}, [9]}, [10]}, [11]}, [12]}, thin-film \(\mathrm {LiNbO_{3}}\) -on-insulator (LNOI) stands out for its ultra-low optical loss [13]}, [14]}, favorable electro-optic effect [15]} and strong optical \(\chi ^{(2)}\)  nonlinearity [16]}, [17]}. Recently, short wavelength operation on thin-film LN platform has been demonstrated at 633 nm [14]}. Other than low loss and high optical nonlinearity, a particular attraction of LN is the ability to reverse the domains periodically for achieving quasi phase matching (QPM) through electric field induced poling [16]}, [20]}, [17]}. Such periodically-poled thin-film LN waveguides provide further enhanced nonlinear coupling due to the high mode confinement and have been recently exploited for efficient photon pair generation [22]}, quantum frequency conversion [23]}, [24]}, second harmonic generation in near-visible wavelengths [14]}, [26]}, [17]}, [28]}. Despite these significant advances, deep visible second harmonic (SH) generation especially at the blue wavelength has remained elusive due to the stringent requirement for ultra short poling period with sub-micron domain structures [29]} and meanwhile maintaining low propagation loss at these wavelengths.
